Here, the term xe2x80x9cdischarge lampxe2x80x9d covers sources of electromagnetic radiation based on gas discharges. The spectrum of the radiation can in this case comprise both the visible region and the UV (ultraviolet)/VUV(vacuum ultraviolet) region as well as the IR (infrared) region. Furthermore, it is also possible to provide a fluorescent layer for converting invisible into visible radiation.
What is involved here are discharge lamps with so-called dielectrically impeded electrodes. The electrodes as such are typically implemented in the form of thin metal strips or layer structures resembling conductor tracks, for example made from conductive silver, at least a portion of which layer structures is arranged on the inner wall of the discharge vessel, for example by means of a printing method such as screen printing or the like. At least a portion of these inner wall electrodes is entirely covered with respect to the interior of the discharge vessel with a dielectric layer which functions during operation of the lamp as a dielectric impediment with reference to the discharge.
If only the electrodes of a single polarityxe2x80x94preferably the anodesxe2x80x94are covered with such a dielectric impeding layer, in the preferred unipolar pulsed operation (WO94/23442) a so-called unilaterally dielectrically impeded discharge is formed which comprises a multiplicity of delta-shaped partial discharges. If, by contrast, all the electrodes, that is to say of both types of polarity, are covered with a dielectrically impeding layer, a so-called bilaterally dielectrically impeded discharge is formed both in the unipolar and in the bipolar operation. In the bipolar operation, for example with ac voltage, or else bipolar pulses (WO94/23442), each electrode alternately undertakes the role both of the anode and of the cathode.
However, it has proved in the case of such lamps that metal ions diffuse out of the electrodes into the dielectric impeding layer and can undesirably influence its properties with reference to its function as a dielectric impediment for the discharge.
In addition, unilateral dielectric impediment is further attended by the problem that metal particles of the unimpeded inner wall electrodes evaporate during the production of the lamp, for example during the burning-in process of the dielectric layer, thermal joining processes and the like, and can possibly be deposited in an uncontrolled fashion inside the lamp. Moreover, in some circumstances the conductivity of the electrode tracks decreases. Said problem is the more pronounced the higher the temperature (in particular, higher than 400xc2x0 C.) and the longer the time during which this temperature prevails. During operation of the lamp, in addition, metal particles can be extracted from the unimpeded electrodes by sputtering processes, and can likewise be deposited on the discharge vessel wall. The metal deposition on the discharge vessel wall leads to a reduction in the luminous flux of the lamp. Moreover, the thickness and width of the typically strip-shaped electrodes influence the current-carrying capacity thereof, and this can become critical, in particular in the case of strong pulsed currents. In addition, the electrode width has the effect on the capacitance of the electrode arrangement which directly influences the dielectrically impeded discharge. Moreover, the striking distance can be partially reduced, something which has a negative influence on the uniformity of the discharge. This holds in particular for the case, explained in more detail in the exemplary embodiments, when the cathode tracks are provided with projections on which the delta-shaped partial discharges attach themselves.
A further functional layer, for example a layer made from a fluorescent material or a mixture of fluorescent materials and/or one or else a plurality of reflecting layers for visible radiation (light) and/or UV radiation can be applied to the dielectric impeding layer and, in general, also to further parts of the inner wall of the discharge vessel. If appropriate, the reflecting layer serves the aim of bringing visible light outside in a specific fashion, that is to say only in a specified preferred direction of the lamp. However, porous layers such as, for example, a layer of fluorescent material provide only a reduced protection against metal ions evaporating or sputtering off out of the electrode tracks. Moreover, the electrode tracks are, in any case, entirely unprotected up until these layers are applied during the burning-in processes.
The geometric shape of the discharge vessel is not subject to any particular restrictions. Tubular or else flat discharge vessels are customary, for example. The latter are suitable, inter alia as so-called flat lamps for backlighting liquid-crystal display screens (LCD). Reference may be made, for example, to DE 197 18 395 C1 or WO 98/43277 with regard to the technical details of such lamps.
It is the object of the present invention to avoid the disadvantages mentioned and to provide a discharge lamp which has an improved design with regard to the long-term performance, in particular also with regard to thinning of the electrodes by diffusion and reducing the influence of the metal electrodes on the dielectric layer.
According to the invention, in the case of the unilaterally dielectric impediment of at least that portion of the inner wall electrodes which is covered with a dielectric impeding layer, is additionally directly covered with a barrier layer, that is to say that the additional barrier layer is respectively arranged between the inner wall electrodes and the dielectric impeding layer. In other words, in this case the arrangement of the layers is as follows, viewed starting from the inner wall of the discharge vessel: electrode layer, barrier layer, dielectric impeding layer. In order to prevent metal particles from evaporating and sputtering off from the electrodes, which was mentioned at the beginning, it is advantageous also to cover the dielectrically unimpeded inner wall electrodes with such a barrier layer.
In the case of bilateral dielectric impediment, according to the invention all the inner wall electrodes, that is to say electrodes of both types of polarity, are directly covered with the barrier layer. The barrier layer is finally followed by the customary dielectric impeding layer.
The barrier layer should cover at least the entire electrode in each case, but can, if appropriate, also be applied xe2x80x9cover the entire surfacexe2x80x9d, that is to say that in the latter case all the electrodes including the discharge vessel wall on which the electrodes are arranged are covered with a single coherent barrier layer. The application of the typically initially pasty barrier layer is performed by standard methods such as spraying, dispensing, rolling, screen printing, or silk screen printing etc.
The barrier layer comprises a dielectric, for example a glass solder, which in addition to preventing evaporation and sputtering away, also prevents metal ions of electrodes from diffusing through the barrier layer into the dielectric impeding layer which is important for the dielectrically impeded discharge. At least partially crystallized or crystallized glass solders, so-called sintered glass ceramics, in particular bismuth borosilicate glass (Bixe2x80x94Bxe2x80x94Sixe2x80x94O), have proved to be suitable in this regard. Further suitable crystallized glass solders are, for example, zinc bismuth borosilicate glass (Znxe2x80x94Bixe2x80x94Bxe2x80x94Sixe2x80x94O), tin zinc phosphate glass (Snxe2x80x94Znxe2x80x94Pxe2x80x94O) and zinc borosilicate glass (Znxe2x80x94Bxe2x80x94Sixe2x80x94O). For the sake of brevity, and in the interest of better terminological delimitation by comparison with the dielectric impeding layer provided for dielectrically impeding the electrodes, the dielectric layer acting as a diffusion, evaporation and sputtering-off barrier layer is also denoted below simply as a (dielectric) barrier layer.
It has proved to be sufficient for the effect according to the invention when the thickness of this barrier layer is of the order of magnitude of at least approximately 1 xcexcm. The thickness of the barrier layer is typically in the range of between 1 xcexcm and 40 xcexcm, preferably in the range of between 1 xcexcm and 30 xcexcm, particularly preferably in the range of between 5 xcexcm and 20 xcexcm. In practice, thicknesses of typically a few xcexcm, for example 6 xcexcm, have proved to be effective. In any case, the thickness of the barrier layer is smaller than the thickness of the impeding layer. Moreover, it is essential that the barrier layer be present in a truly partially crystallized state.
The dielectric impeding layer can be applied to the individual electrodes both in a strip-shaped fashion (for unilateral and bilateral dielectric impediment) andxe2x80x94in the case of the bilaterally dielectrically impeded dischargexe2x80x94xe2x80x9cover the entire areaxe2x80x9d by means of a single coherent barrier layer which covers the entire inner wall electrodes including adjoining parts of the discharge vessel wall.
The selection of the suitable thickness of the dielectric impeding layer is essentially determined by physical discharge requirements and is of the order of magnitude of 50 xcexcm and several hundred xcexcm, in particular in the range of between 50 and 200 xcexcm. The material of the dielectric layer is likewise determined essentially by physical discharge requirements, in particular by the desired dielectric properties, for example dielectric coefficient, electric strength etc. Lead borosilicate glass (Pbxe2x80x94Bxe2x80x94Sixe2x80x94O), for example, is suitable.